Exposure to light is the key mechanism that enables a proper synchronization of the body clock with the solar day cycle. Timing, duration, intensity and spectral composition of light exposure all have impact on the so-called entrainment of a person to a 24-hour circadian rhythm. It has been shown that restorative sleep can only occur in synchronicity with the body clock. For certain people who have a phase shift of their internal body clock relative to the social schedules around them, exposure to bright light at well-defined times can be used to shift their body clock forward or backwards to better align it with their social needs. Also for the treatment of seasonal affective disorder, timed and regular exposure to bright light is an effective means.
The assessment of light levels to which a person is exposed over the course of multiple days or even weeks is an important instrument for the diagnosis of mood and sleep disorders. Existing actigraphy products measure the activity and light exposure with a body-worn device, for example, a wrist-worn device that is used and worn similar to a wristwatch.
Such a wrist-worn device is shown, for example, in US 2008/0319354 A1, showing a system and method for monitoring information related to sleep. The wrist-worn device shown in this document comprises an illumination sensor to provide an information related to the intensity of ambient illumination of the user. The signal of the sensor can be further processed by suitable electronic computing means.
A problem related to the common body worn devices, such as wrist-worn devices, is to detect the presence of an object occluding the light sensor. Such an occlusion may falsify the exposure measurement of ambient light to the user. A typical example for an occluding object is a sleeve of the user's clothing that covers an ambient light sensor integrated into a wrist-worn light sensing device. With the present light sensing devices, it is impossible to determine whether or not the ambient light sensor is occluded by any object, although this information would facilitate the analysis of the exposure data. For example, it would be possible to exclude occlusion periods from the analysis completely. Moreover, it is desired to inform the user of the light sensing device about the occluded state, so he can take counter measures to remove the occluding object from the light sensor to expose it to the ambient light again.